Die casting has been used to form metal castings for many years and produces precise and finished parts which require little or no additional machining.
In die casting, a highly efficient process, a single piston and sleeve arrangement is used for supplying a metal, or metal alloy, to the casting apparatus. The piston is withdrawn to a position which uncovers an opening, or pouring hole, into which molten metal can be poured so as to partially fill a shot chamber formed by the sleeve and piston. The piston is then moved forward so as to force the molten metal into a die cavity. After the metal, or metal alloy, has solidified, the part is ejected from the die. If metal alloys are to be utilized in the die casting process, the materials thereof are alloyed in a separate process and the alloy is then carried to the die casting machine for insertion through the pouring holes of the shot chamber. It is thus mandatory that the metal, or metals, used be readily miscible in the liquid state.
Die casting, however, has not been used to produce metallic composite parts, i.e., parts which comprise a metal, or metal alloy, matrix to which an additional phase is added. The additional phase can be a polymer, a ceramic, glass, or another metal which is immiscible with the matrix metal or metal alloy. These parts are usually manufactured by using powder metallurgy (P/M) techniques in which the component powders are mixed in a blender, following which the blended mixture is pressed in a mold and sintered. Recently injection molding technology, whose application thus far has been limited only to polymeric materials, has been adopted as an alternative means for fabricating metallic, ceramic and cermet parts. In such technique, fine component powders (e.g., in the order of 10 microns) are mixed with thermoplastic binders in a blender until the mixture reaches a slurry-like consistency. The slurry is subsequently injected into a mold at a low hydrostatic pressure using a piston and shot sleeve arrangement. The molded parts are removed from the mold and placed in a furnace, the binder removed, and the parts sintered to higher densities.
While the above mentioned P/M and injection molding techniques are capable of fabricating metallic composite parts, they involve an initial, and separate, mixing or blending operation. In the cases of both conventional P/M or injection molding processes, solid component powders are used as the starting materials, the products being selected so that the composite can meet the requirement of a subsequent sintering step.
It would be desirable to devise an appropriate technique and apparatus for permitting both the mixing of materials in a molten, or a slurry, state to form metallic composite materials (i.e., a combination of metals, or metal alloys, with glass, polymers and/or ceramics) and the casting of articles from said mixed materials in a single apparatus in a substantially continuous operation. In that way the apparatus can both fabricate the desired metallic composite material and cast the finished parts in one overall and cost effective operation, particularly when compared to P/M and injection molding techniques.